Selective etching of silicon-based materials to create silicon pillars is known to be of benefit, for example, in creating anodes for lithium-rechargeable batteries. One such approach is described in U.S. Pat. No. 7,033,936, which is incorporated herein by reference. According to this document, pillars are fabricated by creating a mask by depositing hemispherical islands of caesium chloride on a silicon substrate surface, covering the substrate surface, including the islands, with a film, and removing the hemispherical structures (including the film covering them) from the surface to form a mask with exposed areas where the hemispheres had been. The substrate is then etched in the exposed areas using reactive ion etching and the resist is removed, e.g. by physical sputtering, to leave an array of silicon pillars in the unetched regions, i.e. in the regions between the locations of the hemispheres.
An alternative, chemical approach is described in Peng K-Q, Yan, Y-J, Gao S-P, and Zhu J., Adv. Materials, 14 (2002), 1164-1167, Adv. Functional Materials, (2003), 13, No 2 February, 127-132 and Adv. Materials, 16 (2004), 73-76. Peng, et al. have shown a way to make nano pillars on silicon by a chemical method. According to this method, a silicon wafer, which may be n- or p-type and has the {111} face exposed to solution, is etched at 50° C. using the following solution: 5M HF and 20 mM AgNO3. Pillars are formed at about 20 microns/hr and pillar heights up to 24 microns are reported. Apart from height, no other dimensional data is given, such as mean pillar diameter, packing density, or surface uniformity. The method is only carried on chips of about 1 cm2. The mechanism postulated in these papers is that isolated nanoclusters of silver are electrolessly deposited on the silicon surface in an initial stage. In a second stage, the silver nanoclusters and the areas of silicon surrounding them act as local electrodes that cause the electrolytic oxidation of the silicon in the areas surrounding the silver nanoclusters to form SiF6 cations, which diffuse away from the etching site to leave the silicon underlying the silver nanocluster in the form pillars. It is suggested that metals other than silver, e.g. Ni, Fe, Co, Cr and Mg, especially in the form of nitrate salts, could be used to form silicon pillars.
K. Peng et al., Angew. Chem. Int. Ed., 44 (2005), 2737-2742; and K. Peng et al., Adv. Funct. Mater., 16 (2006), 387-394, relate to an etching method that is similar to that described in the earlier papers by Peng et al but the nucleation/silver nanoparticle deposition step and the etching step are performed in different solutions. In a first (nucleation) step, a silicon chip is placed in a solution of 4.6M HF and 0.01M AgNO3 for 1 minute. A second (etching) step is then performed in a different solution, namely 4.6M HF and 0.135M Fe(NO3)3 for 30 or 50 minutes. Both steps are carried out at 50° C. Pillars of about 6 and 4 microns in height, and about 20 and 20 to 40 nm in diameter, respectively, are reported. Pillars are grown in the {111} direction on the {111} plane. Neither the pillar packing density (surface uniformity), nor the size of silicon chip used are disclosed. The silicon wafer may be n- or p-type. In these papers, a different mechanism is proposed for the etching step as compared to the earlier papers, namely that silicon underlying the silver (Ag) nanoparticles are removed and the nanoparticles gradually sink into the bulk silicon, leaving columns of silicon in the areas that are not directly underlying the silver nanoparticles.
There is a need for higher pillars, increased density of pillars and improved uniformity of pillar distribution. In addition, it would be desirable to increase the speed of pillar formation (etching). Furthermore, because the approaches described in Peng are restricted to the {111} face, the applicability of these approaches is limited.